This invention relates to titanium based alloys and more particularly to titanium aluminide alloys having high strength at elevated temperatures. Alloys of this invention also have sufficient room temperature ductility and fracture toughness to make them useful as engineering materials.
Great technological interest can be found in a titanium aluminide compound containing three titanium atoms per aluminum atom because of its low density and high strength relative to iron or nickel based superalloys or conventional titanium alloys. In the titanium alloy art this compound is designated as Ti.sub.3 Al and is hereafter referred to as trititanium aluminum. Currently, some of the mechanical properties of trititanium aluminum alloys limit their usefulness. Some of the limiting properties are low ductility at room temperature, very little resistance to fracture, and a lack of metallurgical stability at temperatures above 1200.degree. F. Therefore to be used in place of iron or nickel based superalloys, trititanium aluminum alloys must be improved in their room temperature ductility, fracture toughness, and metallurgical stability above 1200.degree. F.
Different operating temperatures in various parts of a gas turbine place increasing demands on the high temperature strength and stability of alloys used in the engines. For example parts in the turbine section may have to operate at temperatures up to 1600.degree. F. while parts in the compressor may operate at 1400.degree. F. with still lower operating temperatures for parts like casings and flow augmentors. Trititanium aluminum alloys that are currently known exhibit a combination of mechanical properties that would make them useful as engineering materials capable of operating at temperatures up to about 1110.degree. F. in lower stressed stationary applications. Therefore, by improving the high temperature strength and stability of trititanium aluminide alloys they can be utilized in more parts of a gas turbine.
The microstructure of titanium alloys and the way they change with a change in composition is well known in the art. When aluminum is added to titanium alloys the crystal form of the titanium alloys change. Small percentages of aluminum go into solid solution in titanium and the crystal form remains that of pure titanium, which is the close packed hexagonal alpha phase. Higher concentrations of aluminum, about 25 to 35%, form the intermetallic compound trititanium aluminum with an ordered hexagonal crystal form called alpha-2. Trititanium aluminum is the material of concern in this application because the titanium aluminum alloys of this invention are an improvement upon prior art trititanium aluminum alloys. Furthermore, the titanium aluminum alloys of this invention have a crystal form that is different from the crystal form of prior art trititanium aluminum alloys.
In pure titanium the alpha phase transforms at approximately 1615.degree. F. to a body centered cubic beta phase. This temperature at which the low temperature alpha phase transforms to the high temperature beta phase is known as the transformation temperature. Certain elements known as alpha stabilizers, stabilize the alpha phase so that the transformation temperature for such alloys is increased above 1615.degree. F. Other elements, such as niobium, stabilize the two phase alpha plus beta region. In titanium alloys the transformation from alpha to beta phase does not occur at a single temperature but over a range of temperatures where both alpha and beta phases are stable. As a result, in titanium aluminide alloys addition of beta phase stabilizers can promote a duplex phase structure of beta phase mixed with alpha or alpha-2 phase depending on the aluminum content.
Limited additions of niobium and other beta phase stabilizers such as molybdenum and vanadium have been shown to improve the room temperature ductility and creep strength of trititanium aluminum alloys, but those improvements have been accompanied by a loss in high temperature strength. Much of the research into titanium aluminides has been for their application in gas turbines. A combination of properties that are desirable in titanium aluminides for gas turbines are high strength and ductility at elevated as well as room temperature, fracture toughness, high modulus of elasticity, creep strength, and forgeability. Therefore, a balance of many properties is needed in a material to be used in gas turbines. However, an undesirable compromise between strength and ductility is necessary when using prior art trititanium aluminum alloys.
Fracture toughness is a measure of resistance to extension of a crack and is measured in units of ksi times square root inch, sometimes abbreviated as ksi.multidot..sqroot.in. The fracture toughness of prior art trititanium aluminum alloys is within the range of 10 to 20 ksi times square root inch. The fracture toughness of prior art trititanium aluminum alloys is well below the 50 to 60 ksi times square root inch fracture toughness of superalloys currently used in the rotating components of gas turbines. Therefore a significant increase in the fracture toughness of trititanium aluminum alloys would be highly desirable to meet the demanding requirements of rotating components in gas turbines.
In U.S. Pat. No. 3,411,901 to Winter it has been shown that titanium aluminide alloys near the composition, in atomic percent, 26.6% aluminum, 9% niobium, 0.8% silicon, with the balance titanium have an optimum combination of ductility and strength. Winter also teaches that when aluminum and niobium content were increased above this optimum composition hardness and strength were found to decrease. Alloys are sometimes hereafter abbreviated by showing, for example, this alloy as Ti-26.6Al-9Nb-0.8Si. All alloy compositions shown herein are in terms of atomic percent.
In the U.S. Pat. No. 4,292,077 to Blackburn et al. it was shown that some mechanical properties were optimized in a trititanium aluminum alloy containing 25 to 27 percent aluminum and 12 to 16 percent niobium. Increasing the niobium content above 16 percent is shown by Blackburn to be undesirable because very little improvement in creep strength was found above that level. Because density is increased when niobium is increased in trititanium aluminide alloys, increasing the niobium above 16 percent produced disadvantageous creep strength-to-density ratios. An industry recognized trititanium aluminum alloy that may be viable for the fabrication of gas turbine components having low fracture toughness requirements is derived from the Blackburn et al. alloy and has the composition Ti-24Al-11Nb.
U.S. Pat. No. 4,716,020 to Blackburn et al. is an improvement upon the '077 patent and discloses the same alloy but with a 0.5 to 4 percent molybdenum addition and a slightly lower niobium addition of 7 to 15.5 percent. Vanadium additions of 0.5 to 3.5 percent can be made to displace part of the niobium. An industry recognized reference alloy from this composition is Ti-25Al-10Nb-3V-1Mo. The teaching from the '020 patent is that molybdenum is a particularly unique addition that improves the high temperature strength and creep strength of the essential Ti-Nb-Al alloy of the '077 patent. However, the increased strength of the Ti-Al-Nb-V-Mo alloy is accompanied by an undesirable reduction in the alloys resistance to fracture at room temperature relative to the Ti-24Al-11Nb alloy.
Both Winter and Blackburn et al. found limited niobium additions of up to 16 atomic percent optimize the properties of aluminum alloys. Blackburn et al. then made improvements in the high temperature strength and creep rupture properties of Ti-Al-Nb alloys in the '020 patent, not through modification of the niobium content, but through the addition of molybdenum.
Contrary to the findings of Winter and Blackburn et al. we have found that high temperature strength and fracture toughness of titanium aluminide alloys are improved beyond the levels of these prior art alloys by increasing niobium contents substantially above 16 atomic percent.
The alloys of this invention contain titanium and aluminum contents typical of trititanium aluminum alloys and trititanium aluminum alloys are known to have the alpha-2 crystal form as their normal low temperature phase structure. Alloys of this invention also contain a substantially increased percentage of beta phase stabilizing niobium over the Winter and Blackburn et al. alloys. Since niobium is a beta phase stabilizer its presence in the trititanium aluminum alloys would be expected to preserve some beta phase in the low temperature alpha-2 phase of trititanium alloys. For example, the preferred microstructure of Blackburn et al. in their trititanium aluminum alloys containing niobium is a Widmanstatten structure characterized by an acicular alpha-2 phase mixed with beta phase lathes. Surprisingly the increase in niobium in the alloys of this invention substantially above 16 atomic percent did not lead to an increase in the amount of beta phase with a decrease in the amount of alpha-2 phase. Instead a new microstructure was discovered in the alloys of this invention having an ordered orthorhombic crystal form rather than the hexagonal alpha-2 or body centered cubic beta crystal forms that are known to be present in trititanium aluminum alloys. Beta, ordered beta or alpha-2 phase may be present in the alloys of this invention but an important contribution to the improved properties in the alloys of this invention is believed to be due to the presence of the orthorhombic phase. The ordered orthorhombic phase is believed to form the intermetallic compound Ti.sub.2 AlNb.
Therefore, it is an object of this invention to provide titanium aluminide alloys containing a substantial portion of an orthorhombic crystal form comprising at least 25% of the volume fraction of their microstructure.
Another object of this invention is to provide titanium aluminide alloys containing niobium additions substantially above 16 atomic percent and having superior tensile strength at elevated temperatures up to 1500.degree. F. while retaining sufficient ductility at room temperature and good fracture toughness so they can form useful engineering materials.